rm: expand the index only when necessary

Remove the `ensure_full_index()` method so `git-rm` does not always
expand the index when the expansion is unnecessary, i.e. when
<pathspec> does not have any possibilities to match anything outside
of sparse-checkout definition.

Expand the index when the <pathspec> needs an expanded index, i.e. the
<pathspec> contains wildcard that may need a full-index or the
<pathspec> is simply outside of sparse-checkout definition.

Notice that the test 'rm pathspec expands index when necessary' in
t1092 *is* testing this code change behavior, though it will be marked
as 'test_expect_success' only in the next patch, where we officially
mark `command_requires_full_index = 0`, so the index does not expand
unless we tell it to do so.

Notice that because we also want `ensure_full_index` to record the
stdout and stderr from Git command, a corresponding modification
is also included in this patch. The reason we want the "sparse-index-out"
and "sparse-index-err", is that we need to make sure there is no error
from Git command itself, so we can rely on the `test_region` result
and determine if the index is expanded or not.

Helped-by: Victoria Dye <vdye@github.com>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rm.c

@@ -296,8 +296,9 @@ int cmd_rm(int argc, const char **argv, const char *prefix)
 
 	seen = xcalloc(pathspec.nr, 1);
 
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(&the_index);
+	if (pathspec_needs_expanded_index(&the_index, &pathspec))
+		ensure_full_index(&the_index);
+
 	for (i = 0; i < active_nr; i++) {
 		const struct cache_entry *ce = active_cache[i];